refactor: Implement Singleton pattern for DatabaseManager

Refactor the database handling to use a Singleton pattern for `DatabaseManager`.

- A single, module-level instance `db_manager` is created in `core/database.py` to ensure one database connection is used throughout the application.
- `MediaRepository` and `MainController` are updated to use this shared instance instead of creating their own.
- This simplifies dependency injection and prevents potential issues with multiple database connections.
- Also, update `review.md` to reflect the progress.

controllers/main_controller.py

@@ -3,7 +3,7 @@ from PySide6.QtCore import Slot
 from PySide6.QtGui import QPixmap
 from PySide6.QtWidgets import QPushButton
 
-from core.database import DatabaseManager
+from core.database import db_manager
 from core.media import MediaRepository
 from core.camera.camera_manager import CameraManager
 from ui.widgets.color_list_widget import ColorListWidget
@@ -14,8 +14,8 @@ from ui.widgets.split_view_widget import SplitView, CameraPlaceholder, ViewWithO
 class MainController:
     def __init__(self, view):
         self.view = view
-        self.db = DatabaseManager()
-        self.media_repo = MediaRepository(self.db)
+        self.db = db_manager
+        self.media_repo = MediaRepository()
         self.camera_manager = CameraManager()
 
         # --- UI Widgets ---